1. Field of the invention
The present invention relates to automotive vehicle transmission systems and is concerned with that type of such system which comprises two or more compounded epicyclic gearsets, two elements of one of which are connected to rotate with respective elements of the other and connected to rotate with one element of each of which is a respective electric motor/generator unit, the electrical stator connections of which are connected together, whereby power is transferred between the two motor/generator units. Such a transmission system is referred to as of power split or torque split type because the power passing through it is transmitted in part mechanically, i.e. through the two mechanical connections, and in part electrically, i.e. through the electrical connections between the two motor/generator units, the ratio of the two components varying with speed and load. Such transmission systems inherently have a continuously or steplessly variable transmission ratio and are therefore highly advantageous because they provide at all times the gear ratio which is the most appropriate to the user's decision relating to the trade off between optimising fuel economy and maximising acceleration of the vehicle.
2. Description of the Related Art
U.S. Pat. No. 5,558,589 discloses a transmission system of this general type which includes an input shaft which is connected to an engine and, via a first clutch, to the annulus of a first planetary gearset. The annulus meshes with a plurality of planet wheels, which are mounted on a common carrier and mesh with a sunwheel. The annulus is connected to rotate with the planet carrier of a second planetary gearset. The carrier of the first gearset is also connected to rotate to the annulus of the second gearset. The carrier of the second gearset carries planet wheels which mesh with the annulus and the sunwheel of the second gearset. The carrier of the first gearset is connected to a drive shaft which is connected to an output shaft through a second clutch. The sunwheel of the second gearset is also connected to the output shaft through a third clutch. A battery is connected via a controller to first and second motor/generators which are connected to rotate with the first and second sunwheels, respectively.
The transmission system disclosed in the prior U.S. patent is of two regime type. In the first regime, the first and third clutches are engaged and the first motor-generator operates as a generator and its output is fed to the second motor/generator which operates as a motor. The vehicle is accelerated from rest to a speed of about 5 mph in the first regime. The vehicle controller then applies the battery power to the second motor/generator and the speed of the first motor/generator progressively falls to zero, at which point the full power of the engine and the battery are accelerating the vehicle. The transmission is then switched into the second regime by disengaging the third clutch and engaging the second clutch. Acceleration then proceeds up to the desired speed.
As mentioned above, this known transmission system is of two mode or two regime type, that is to say the output shaft is connected selectively to the carrier of the first gearset or the sunwheel of the second gearset by respective clutches. This is necessary in order to be able to obtain the full speed range at the output shaft which is generally necessary for all vehicle applications. The input shaft is connected to the annulus of the first gearset by means of a further selectively operable clutch and whilst this clutch is always engaged during forward movement of the vehicle, it has to be disengaged if the vehicle is to move rearwardly because rearward movement is not produced by energy transmitted from the associated vehicle engine to the input shaft but is instead produced by energy transmitted from a battery with which the transmission system must be provided.
The use of two or more regimes and the necessary associated clutches in a transmission system is, however, associated with a number of disadvantages. Thus the number of components and overall complexity is considerably increased by contrast with a single regime system. Although acceleration can be substantially maintained whilst switching from one regime to another, acceleration is interrupted and this results in jerkiness which is undesirable and has proved impossible to eliminate. Debris necessarily produced by the clutches means that the transmission oil has to be changed relatively frequently.
Nearly all automotive transmission systems of this type which have been proposed have necessarily included two or more regimes because the ∅max ratio of each regime is relatively low. The ∅max ratio is defined as the ratio of the maximum and minimum possible output speeds at any given value of power throughput and ∅max is the value of this ratio at the maximum or rated power of the transmission system. The result of this is that two or more regimes have been necessary in order to obtain a sufficiently broad range of output speeds to render the transmission system usable for automotive applications.
A paper entitled “Power-Dividing Transmissions” by D. L. Bedingfield published in “Machine Design”, 25th Jul. 1957 discloses a power split transmission in which the input is constituted by the annulus of the first epicyclic gearset and the output is constituted by the planet carrier of the second epicyclic gearset. The planet carrier and annulus of the first epicyclic gearset are connected to the annulus and sunwheel, respectively, of the second epicyclic gearset. The rotors of the two motor/generator units are connected to rotate with the sunwheel of the first gearset and the annulus of the second gearset, respectively.
The two mechanical connections between the two gearsets necessarily “cross over” in this transmission system and this results in considerable mechanical complexity. This mechanical complexity is further increased by the fact that the two motor/generators are situated between the two epicyclic gearsets which means that the high torque shafts necessarily pass through one of the motors. Also the output speed is too low for automotive applications and the output torque too high. Furthermore, it is found that the two motor/generator units effectively “work against” one another which means that there are very high levels of recirculating power around the loop constituted by the two gearsets and the two mechanical connections between them. In practice, the magnitude of the power that is recirculated is found to be substantially in excess of the input power and this means that the components of the two gearsets must be substantially heavier and thus more expensive than would otherwise be the case and/or that the service life of the transmission is unacceptably short. In practice the transmission system disclosed by Bedingfield is unacceptable, both commercially and technically.
It is therefore the object of the invention to provide an automotive transmission system of the type referred to above, particularly for road vehicle use, which can provide a sufficiently broad range of output speeds in a single regime, that is to say does not have any clutches and thus does not suffer from the disadvantages referred to above of such clutches, and which does not suffer from the disadvantages suffered by the transmission system disclosed by Bedingfield.